In motor vehicles, there has been increasing use of automatic stepped transmissions with at least one automated friction clutch as a startup element, in which the gear selection, the triggering of shift operations, the engaging and disengaging of gear steps, and the engaging and disengaging of the friction clutch are automated; that is, these operations occur by evaluating operating parameters in a transmission control device and the drive assigned to the control.
For startup from standstill, the friction clutch can be a passively engageable single or multi-disc dry clutch, or an actively engageable multi-disc clutch for bridging the speed difference between the engine speed and the transmission input speed (speed at the input shaft of the stepped transmission) in slipping operation, until the motor vehicle has accelerated and synchronous running arises at the friction clutch and the clutch can be completely engaged.
The startup-dependent slipping operation represents a large thermal and mechanical load for the friction clutch, which increases with the value of the startup torque, the value of the slip speed and the duration of the slipping phase, and which forms an essential parameter for determining the startup gear.
In addition, the engine torque (startup torque) that can be delivered by the drive engine must be sufficiently high to overcome the stationary drive resistance of the motor vehicle (rolling resistance plus incline resistance) with the overall transmission ratio determined by the selected startup gear and the efficiency of the drive train, as well as to enable additionally at least a minimal startup acceleration of the motor vehicle (dynamic drive resistance).
It must also be considered during startup that active output drive-side power take-offs, that is, power take-offs disposed at the transmission and/or the axle transmission that reduce the engine torque which can be used for startup, can be considered as a fictonal additional resistance for the determination of the startup gear. In contrast, auxiliary consumers driven directly by the drive engine such as an electric generator, a servo pump of a servo steering, and an air conditioning compressor of an air conditioning system, as well as active drive-side power take-offs, that is, disposed directly at the drive engine, already reduce at the source of the torque, the engine torque that can be delivered by the drive engine to the friction clutch and that is available for startup.
In order to avoid many sequential shift procedures associated in each case with an interruption of tractive force, it is generally desirable to perform the startup of a motor vehicle in as high a gear step as possible, however without thermal overload of the friction clutch in the process.
Thus, a method for determining a startup gear is known from the documents DE 198 39 837 A1 and U.S. Pat. No. 6,953,410 B2, with which the highest possible startup gear is determined from the present drive resistance of the motor vehicle and the available engine torque of the drive train so that the expected duration of slipping of the friction clutch during the startup and/or the thermal energy created in friction clutch in slipping operation do not exceed predetermined limit values.
The document U.S. Pat. No. 7,220,215 B2 describes a commercial vehicle with a control device with which the highest possible startup gear is determined so that the maximal engine torque that can be generated by the drive engine with the idle speed is attained for the startup, and in the process the thermal energy created in the friction clutch does not exceed a predetermined limit value.
The typical method process for determining a startup gear as well as the startup torque and the startup speed of the drive engine can be found in DE 10 2007 019729 A1. An alternate method for determining a startup gear according to the document DE 10 2007 031 725 A1, with which the laborious use of characteristic curves and characteristic maps is avoided in its application, provides that a minimum startup transmission ratio is calculated from the present drive resistance, the available engine torque and an assumed minimum startup acceleration, and the highest possible startup gear is determined therefrom.
Particularly in the case of commercial vehicles, the drive engines are usually designed as diesel engines that can be loaded by a turbo-charger, which have a specific load build-up characteristic. As described in detail in the document DE 10 2008 054 802.2, which was previously unpublished, and which describes a method for controlling an automatic stepped transmission depending on the dynamic operating characteristics of a turbo-charged internal combustion engine, a turbo-charged internal combustion engine can spontaneously, that is with high torque gradients, only reach an intake torque lying below the full load torque. A further increase of the engine torque is possible, although with low torque gradients, only briefly above a boost threshold speed, after which the turbo-charger creates a significant increase of the charge pressure and thus the engine torque. Thus aside from the idle speed, cut-off speed and the full load torque characteristic curve, the dynamic behavior of a turbo-charged internal combustion engine is also determined by the boost threshold speed and the intake torque characteristic curve as well as by the present torque gradients, at least in certain regions.
With the previously known methods for startup control of motor vehicles, the dynamic operating characteristics of the drive engines were considered only indirectly using characteristic curves and characteristic maps that are laborious to apply and correspondingly inexact. This can have the consequence that the startup torque necessary for startup is set with an engine speed (startup speed) that is too high, which causes an increased thermal and mechanical load of the friction clutch due to the high slipping speed. Likewise, an engine speed (startup speed) that is too low for the startup can be set, with which the startup torque required for startup cannot be spontaneously generated by the drive engine such that either the startup procedure must be aborted, or a shift into a lower startup gear must occur during the startup, or the startup torque is achieved only greatly delayed with prolonging the slipping phase of the friction clutch.